Sticky visibility setting

Dokku: https://sticky.dokku.hypothes.is/

- AnnotationController.save() saves the used Privacy into localStorage
- AnnotationController.edit() sets (if given) the stored privacy for root annotations
- Minimal wrapper around localStorage access as a service called: localStorageHelpers

Fix #1299 

> :rotating_light:
>
Yes, sorry. Left there by accident. Removed.